Worm wheel gearboxes are well known in the art. Such gearboxes are especially useful where low speed and high torque is desirable. Within these gearboxes, a worm gear engages a bull gear. Often, the worm gear is disposed on an input or drive shaft and the bull gear is linked to an output shaft. These worm wheel gearboxes are commonly used, for example, in agricultural automatic irrigation systems and potato pilers.
Large scale agricultural automatic irrigation systems typically comprise a system of sprinkler heads disposed on a truss or frame network. The trusses are linked together, enabling such irrigation systems to stretch to lengths of a thousand yards or more. The ground supports of the trusses are each disposed on a wheel or pair of wheels so that the entire irrigation system can be moved across a field. It is common for such irrigation systems to rotate about a center point, resulting in a substantially circular irrigating footprint. However, such irrigation systems may also be adapted to move in a straight line across rectangular fields.
Each wheel or pair of wheels supporting and transporting the irrigation system is driven by a dedicated motor. The motor drives a shaft which acts as the input shaft to a worm wheel gearbox. A worm gear disposed on the input shaft is engaged with a bull gear within such a gearbox. The bull gear is linked to an output shaft. The output shaft has an output flange which connects to the driven wheel. Rotation of the input shaft is thus transmitted via the gearbox to the output shaft, driving the wheels of the irrigation system. A typical automatic irrigation watering system has a number of such support wheels and each wheel or pair of wheels typically is driven by a dedicated motor and worm wheel gearbox as described. Worm wheel gearboxes are especially advantageous in this environment because once the drive motor stops, the worm and bull gear combination will allow very little backlash or coasting. Thus, the irrigation system will remain in its position even if it is on a hill.
The farm environment tends to be wet, muddy, silty and dusty. Thus, these gearboxes are sealed to prevent contamination of the gearbox contents.
As discussed above, worm wheel gearboxes are also commonly used in potato pilers. A potato piler comprises a conveyor disposed on a wheeled frame. To enable even piling of potatoes, the conveyor must be moved short, precise distances during operation. Potato pilers thus typically comprise a motor which rotatably drives a shaft and a worm wheel gearbox that transmits the shaft rotation to drive the wheels of the potato piler. This enables the potato piler to be moved short, precise distances when piling potatoes.
The need often arises to tow the units described above. For example, some farmers, in order to reduce costs, will use one irrigation system for a number of fields. Thus, it is necessary to tow the entire irrigation system from one field to another. Similarly, once a warehouse has been filled with potatoes, the potato piler must be transported to another warehouse. Thus, there is a need to tow the potato piler.
Worm wheel gearboxes are not adapted for easy towing. The gear ratios are best adapted for high-torque, low-speed situations. Accordingly, there is a need for a worm wheel gearbox that can be made to freewheel in order to ease towing.
The industry's standard solution to this towing problem is to attach a freewheeling hub and flange onto the output shaft of the worm wheel gearbox. This freewheeling flange is then connected to the driven unit's wheel. When the unit is being towed, this freewheeling flange allows the wheels to spin free.
To engage the wheel with the gearbox, a pin is pushed through aligned holes in both the output flange and the freewheeling flange so that the freewheeling flange and the wheel, which is attached thereto, are turned with the output flange. This approach has many limitations. First, the extra parts make this towable gearbox more expensive to manufacture than a non-towable gearbox. Also, it requires at least one loose part (pin) which can be lost. This approach also exerts the entire driving force of the motor and gearbox on the pin, resulting over time in pin failures. Maintenance costs are high, as is downtime and the risk of losing crops due to missed irrigation.
Another approach to solving the towing problem uses locking hubs similar to those used in four-wheel drive vehicles. However, this construction has proven too complex and expensive to be practical in this application.